Physical Study Of The Ultra-high Brightness X-ray Free Electron Laser

Conventional laser based on the electron bound state,which is confined by the band of electron transition energy,is hardly to generate lasers in the X-ray range.However,the free electron beam passing through the periodic magnets is not limited to generate short wavelength radiation.Since 1976,the concept of free electron laser(FEL)is introduced,FEL has been developed rapidly.By 2009,the first hard X-ray free electron laser was built,which marked the technology of the FEL has been matured.In view of the wide applications of XFEL in science and technology,lots of XFEL facilities are under construction in developed countries.At present,most of the XFEL facilities are built on self-amplified spontaneous radiation(SASE)scheme,but there are still some problems in SASE-FEL.For the SASE scheme,the radiation pulse is developed from the localized shot-noise which is uncorrelated in the phase causing the final output radiation pulse comprised of many longitudinal modes,that is,there are spikes in the spectrum,resulting the brightness decrease of the spectrum.To improve the spectral brightness and coherence of the XFEL,scientists have made extensive exploration,lots of schemes have been proposed.For example,high-gain harmonic generation scheme(HGHG)and echo-enabled harmonic generation scheme(EEHG)based on the external seed-laser,besides,the self-seeding scheme(SXRSS or HXRSS)which is based on the crystal to filter one single color to amplify from the SASE pulse,and the high-brightness SASE scheme(HB-SASE)according to the slippage effect between the electron beam and radiation pulse.This dissertation is to study the problems existing in the SASE-FEL,that is,how to improve the spectral brightness and coherence of SASE-FEL,to make the XFEL has wider applications.For the single-stage HGHG based on the external seed-laser,it is difficult to generate short wavelength radiation,because the up-conversion harmonic number is limited(~15)by the energy spread of the electron beam.Thus,we study using the single-stage EEHG to generate the radiation that the up-conversion harmonic number is 150,focusing on the theory of high order mode of EEHG-150.Besides,the incoherent synchrotron radiation(ISR),coherent synchrotron radiation(CSR)and intra-beam scattering(IBS)effects all have been considered.Finally,1.77 nm nearly Fourier transform limited FEL pulse is obtained by numerical simulation which will be used to guide the future experiments of the ultra-high EEHG scheme.To overcome the low efficiency of single-stage HGHG in transformed harmonic number and the stability of cascaded HGHG.We propose a new scheme HB-HGHG to generate high brightness,highly coherent and ultra-short soft X-ray in water window.CHG is used to generate coherent signal,and then the signal is amplified by the fresh part of the electron beam in the second stage.The output peak power of HB-HGHG is one order of magnitude larger than the two-stage cascaded HGHG and conventional EEHG.At present,the XFEL facility is expensive for the high energy of the accelerated electron beam.To reduce the energy of electron beam and maintain the saturation power.A new scheme named CHG-HL is proposed,which is based on harmonic operation.Firstly,CHG is also used to generate short wavelength signal,however,the coherent signal is resonant on the harmonic frequency of the undulator in the second stage.Using the CHG-HL scheme,the 3nm and 1.2GW radiation pulse is generated by the 0.84GeV electron beam which is much lower than 1.6 Ge V.China will build a hard XFEL facility(SCLF)with high repetition rate(1MHz),based on superconducting linac.But,there is no suitable scheme for XFEL to generate highly coherent hard X-ray with high repetition rate,even the self-seeding scheme is just suitable for the low repetition rate XFEL because of thermal loading effect.To seek the full potential of high repetition rate XFEL facilities,we study the HB-SASE scheme,and a new inversed sequence is proposed to maintain the bunching of the electron beam.Meanwhile,a new magnetic compression structure is designed to delay the electron beam to get enough slippage length.Two-color hard X-ray FEL is useful for the forthcoming pump-probe experiments.Combining with the split delay line(SDL),we have proposed four cases to generate time tuned two-colors at the European XFEL facility.Especially,the HXRSS scheme with a crystal which can rotate in two directions with pitch angle and yaw angle,can generate two-colors comprised of 10~111 photons for each color.Besides,with the fresh bunch technique,we can get the spacing energy between the two-colors to 1000eV,these all will open up new fields for science and technology in the future.